1. Technical Field
This invention relates to multilayer ceramic integrated circuit packaging substrates, and more particularly to a method of and apparatus for forming multilayer ceramic integrated circuit packaging substrates from large area ceramic green sheets.
2. Background Art
The use of multilayer ceramic substrates for mounting and interconnecting integrated circuit devices is well known in the art as indicated by an article entitled "A Fabrication Technique for Multi-Layer Ceramic Modules", by H. D. Kaiser et al., Solid State Technology, May 1972, page 35-40. Multilayer ceramic substrates are typically formed from ceramic green sheets. An array of multilayer ceramic substrate layer sites is formed on a green sheet by punching a predetermined pattern of holes (vias) for each layer site and filling the vias with a conductive metal, typically molybdenum. A screened surface metallization pattern interconnects the metal filled vias in a layer site. Up to thirty or more green sheets are stacked, laminated and sintered to form a solid ceramic stucture. The solid ceramic structure is then cut or diced into individual multilayer ceramic substrates.
The above described process inherently limits the maximum green sheet size which may be employed to fabricate multilayer ceramic substrates. It would be desirable to use a large area green sheet having many layer sites thereon, because of the reduced cost per layer site for the above described green sheet fabrication, punching, and metallization steps. However, since the punching and metallization steps produce dimensional distortions in the green sheet, the number of layer sites that may be formed on a given green sheet must be limited in order to ensure alignment of vias in corresponding layer sites on two or more superimposed green sheets. Accordingly, via misalignment in corresponding layer sites of superimposed green sheets prevents the use of large area green sheets in the above described multilayer ceramic substrate fabrication process.
Another process for manufacturing multilayer ceramic substrates is disclosed in U.S. Pat. No. 3,546,776 to A. R. Rodriguez et al., wherein a plurality of green sheets, each of which has a plurality of metallized patterns repeated thereon at spaced intervals, are arranged in a stack so that the patterns of one sheet are vertically aligned with the different patterns on the other sheets. The stack of green sheets is punched into a die to form the layers of a multilayer ceramic substrate. The Rodriguez et al. technique also inherently limits the size of the green sheet which may be employed. For large area green sheets, dimensional tolerances preclude the vertical alignment of the patterns on one sheet with the different patterns on the other sheets. The Rodriguez et al. reference also discloses that individual layer sites may be punched into the die cavity from individual sheets; however, the use of a separate green sheet for each layer site drastically limits throughput and unit cost per multilayer ceramic substrate. Accordingly, the prior art has not suggested a method and apparatus for forming multilayer ceramic substrates from green sheets of unlimited size.